The present invention is related to a reflecting surface for synthesis of reflected wavefronts therefrom for use in reflecting antennas and mirrors, for example. More particularly, the present invention is related to a system and method of making and using such a reflecting surface that is particularly useful for reflecting millimeter-wave frequencies.
Reflecting antennas and mirrors, such as those used in beam-waveguide systems, tend to be difficult and expensive to build for millimeter-wave frequencies because the mechanical tolerances required to achieve the best signal are difficult to attain. For example, as a general rule the reflecting surface of a parabolic reflector must conform to the ideal paraboloid to within approximately one-fiftieth of a wavelength. At a frequency of 100 GHz, this corresponds to a tolerance of approximately 2 mils (about 50.8 xcexcm). As the frequency and/or the size of the reflector increases, holding the required tolerance becomes more difficult. A regular curved surface, such as a paraboloid or hyperboloid, is particularly difficult to manufacture to a high degree of precision.
As difficult as it can be to manufacture a regular curved surface, some applications require an irregular curved surface in order to produce a desired far-field pattern, or an irregular reflecting surface (in a beam-waveguide system, for example) to correct the phase of the incident beam. Depending on the frequency and the required degree of irregularity, such a curved surface may be cost prohibitive to machine and in some cases impossible to manufacture with current manufacturing techniques.
Flat Parabolic Surface (FLAPS) antenna technology attempts to solve this problem by using an array of dipoles separated from a ground plane by a dielectric layer. The local phase shift imparted to the wave reflected from the FLAPS surface is determined by the geometry of nearby dipoles. By proper variation of the dipole geometry and spacing as a function of location on the FLAPS surface, the properties of a conventional curved reflecting antenna can be emulated.
Unfortunately, however, dielectrics generally are not environmentally rugged, and must be protected from the weather. In addition, in some applications (experimental inertial-confinement fusion reactors, for example) the beam may carry more than a megawatt of power at frequencies exceeding 100 GHz. Dielectrics tend to be lossy at millimeter-wave frequencies, and are poor conductors of heat, both of which are serious disadvantages in high-power applications. Therefore, use of a dielectric layer to support the dipoles in a FLAPS system generally precludes its use in high power applications.
Unlike prior systems, the present invention provides a reflecting surface in the form of a plate having cavities of varying dimensions and/or spacing to achieve a desired local phase shift across the reflecting surface, thereby eliminating the need to use dielectric materials. The surface of the plate can be flat or curved. A plane wave incident on the plate undergoes a shift in phase upon reflection, with the local phase shift depending on the dimensions and spacing of the cavities. By properly choosing the cavity dimensions as a function of position on the plate, the wavefronts reflected from the plate can be made to mimic a wavefront reflected from an equivalent curved reflector. In other words, the present invention provides a reflecting structure having a desired surface geometry that can emulate the electromagnetic behavior of an arbitrarily curved surface. For example, a reflecting structure that emulates a parabolic reflector can be embedded in a cylindrical surface, e.g., the skin of an aircraft. Reflecting antennas and mirrors based on this technology offer significant advantages in cost and performance over their conventional-shape counterparts.
More particularly, the present invention provides a wavefront transformer suitable for transforming an incident electromagnetic wavefront having a given shape to a reflected wavefront having a different shape. The wavefront transformer includes a substrate having a conductive surface for reflecting the incident electromagnetic energy, and a plurality of openings in the conductive surface. Each opening is formed by a respective one of a plurality of discrete cavities extending from the conductive surface, and has a selected position on the conductive surface with respect to the focal point to induce a propagation phase shift over the distance to the focal point. Each cavity also includes a local phase shift in the reflected electromagnetic energy as a function of a selected dimension of the cavity. The combined propagation phase shift and local phase shift from the plurality of cavities places the reflected electromagnetic energy in phase at the focal point.
Other features encompassed by the present invention include a wavefront transformer wherein the substrate is a metal plate; wherein the plate is substantially flat; wherein the plate includes a first plate overlying a second plate, wherein the first plate has a plurality of through-holes therein that form the cavities and the second plate forms a flat bottom surface of the cavities; wherein the plate has a substantially uniform thickness; wherein one or more properties of the cavities varies with position with respect to the focal point; wherein the properties that vary include dimensions of the cavities and spacing between neighboring cavities; wherein the dimensions of the cavities include cross-sectional dimensions that include one or more of width, depth and radius; wherein the plurality of cavities form a periodic array; wherein only the positions of the cavities and the selected dimension of the cavities varies, and the dimension of each cavity is selected such that the total phase shift at the focal point of an electromagnetic wave reflected from each cavity is equal, so that             φ      ⁡              (        r        )              =                  φ        ⁡                  (          0          )                    +                                    2            ⁢            π                    λ                ⁢                  (                                                                      r                  2                                +                                  f                  2                                                      -            f                    )                      ,
where r is the distance of the cavity from a reference point in the plane of the conductive surface, xcfx86(r) is the local phase shift imposed on an incident electromagnetic wave at r by the flat reflecting surface, f is the focal length of the reflector, xcex is a desired wavelength of the reflected electromagnetic energy, and xcfx86(0) is the local phase shift imposed on an incident electromagnetic wave by a cavity at the reference point having a dimension a(0,0).
Other features include a wavefront transformer having a focal length of about four and a half inches (about 11.4 cm); wherein a dimension of the central cavity, a(0,0), is a radius of a circular opening formed by a cylindrical cavity; wherein a(0,0) is about 44.5 mils (about 254 xcexcm); wherein the cavity dimension is selected for frequencies greater than about 20 GHZ; wherein the cavity dimension is selected for a frequency of about 95 GHz; wherein the cavities have a uniform depth of about 100 mils (about 2.54 mm); wherein the nearest-neighbor distance between adjacent cavities is uniform; wherein the nearest-neighbor distance between adjacent cavities is about 105 mils (about 2.67 mm); wherein the cavities have a depth that is less than a local thickness of the plate; wherein the openings are circular; wherein the cavities are cylindrical; and wherein the plurality of cavities are arrayed in an equilateral-triangular arrangement.
The present invention also provides a reflector suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, including the wavefront transformer, and an antenna including the reflector and a waveguide feed located at the focal point.
The present invention also provides a method of making a reflector suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, the wavefront transformer having a substrate with a conductive surface for reflecting the incident electromagnetic energy, and a plurality of openings in the conductive surface, each opening formed by a respective one of a plurality of discrete cavities extending from the conductive surface. The method includes the following steps: selecting a dimension of each cavity as a function of a propagation phase shift and a local phase shift created by the cavity at a desired distance from the focal point, and forming the cavities in a conductive surface, wherein the dimension of each cavity is selected such that the local phase shift imposed on an incident electromagnetic wave is       φ    ⁡          (      r      )        =            φ      ⁡              (        0        )              +                            2          ⁢          π                λ            ⁢              (                                                            r                2                            +                              f                2                                              -          f                )            
where r is the distance of the cavity from a reference point on the conductive surface, f is the focal length of the wavefront transformer, xcex is a desired wavelength of the reflected electromagnetic energy, and xcfx86(0) is the local total phase shift imposed on an incident electromagnetic wave at the reference point by a cavity having a dimension a(0,0).
Other features encompassed by the present invention include a method wherein forming the cavities includes forming the cavities in an equilateral-triangular arrangement; forming through-holes in a first plate and mounting the first plate on a backing plate that forms a solid bottom surface for each hole; machine reaming; and using electronic discharge machining.
The present invention also provides an antenna suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, including a geometrically flat wavefront transformer plate having a conductive surface and a waveguide feed positioned at the focal point suitable to receive the reflected electromagnetic energy. The wavefront transformer plate further includes a plurality of discrete cavities opening in the conductive surface, the dimensions of each cavity varying as a function of the position of the cavity on the plate with respect to the focal point to induce a local phase shift on the incident wave of electromagnetic energy as the electromagnetic energy is reflected, and the cavities being spaced with respect to adjacent cavities to enable the wavefront transformer plate to focus the reflected electromagnetic energy at the focal point such that electromagnetic energy reflected from the wavefront transformer plate is in phase at the focal point.
According to one embodiment of the antenna, the cavities are arrayed in an equilateral-triangular arrangement.
The present invention also provides a reflector suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, including means for focusing an incident plane wave of any polarization at the focal point.
According to one embodiment of the reflector, the means for focusing includes a substrate having a conductive surface for reflecting the incident electromagnetic energy, and a plurality of discrete cavities having openings in the conductive surface, each cavity forming part of at least one equilateral-triangular arrangement of cavities.
In accordance with an exemplary embodiment of the invention, a reflector is formed of a geometrically flat plate, and only the positions of the cavities and the selected dimension of the cavities are varied across the reflector. The dimension of each cavity is selected such that the portion of the incident wave reflected by each cavity arrives at the focal point with the same phase (within a multiple of 2xcfx80 radians). Mathematically, this means that the phase shift imposed on the reflected wave by a cavity a distance r from a reference point in the plane of the conductive surface is       φ    ⁡          (      r      )        =            φ      ⁡              (        0        )              +                            2          ⁢          π                λ            ⁢                        (                                                                      r                  2                                +                                  f                  2                                                      -            f                    )                .            
In this equation, f is the focal length of the reflector, xcex is a desired wavelength of the reflected electromagnetic energy, and f(0) is the local phase shift imposed on the reflected wave by a cavity at the reference point having a dimension a(0,0).
A reflector produced in accordance with the present invention does not suffer from the same limitations as prior systems and can be used in place of a curved mirror without sacrificing power carrying capacity. Moreover, the reliance of the reflector on cavities to form the reflected wavefront rather than the curvature of the surface offers flexibility in design, as well as cost advantages, particularly in manufacturing, that otherwise would not be available. These advantages are further enhanced by the improved environmental ruggedness of the reflector.
Accordingly, the present invention provides reflecting surfaces for synthesis of reflected wavefronts of desired shapes, and the reflecting surfaces may have geometries that are independent of the geometry of the reflected wavefront. In other words, a flat plate can produce a parabolic reflected wavefront, for example.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this embodiment being indicative, however, of but one of the various ways in which the principles of the invention may be employed.